Sodium cobalt bronzes have recently found use in room temperature batteries with organic liquid electrolytes and sodium, sodium alloy or sodium composite anodes. These materials exist in several phases, depending upon composition (x in Na.sub.x CoO.sub.2) and the method of preparation. Most notably, the P.sub.2 phase Na.sub.x CoO.sub.2 has been shown to undergo electrochemical intercalation and de-intercalation of sodium reversibly over a range of x=0.3-0.9 without undergoing structural changes, and thus is particularly well-suited for use in secondary batteries.
The general procedure for synthesizing sodium cobalt bronzes is to heat an intimately mixed sample of Co.sub.3 O.sub.4 and a sodium salt (e.g. Na.sub.2 O.sub.2 or Na.sub.2 CO.sub.3) in air or oxygen to 400.degree. C. or higher for several hours. Compounds ranging from about x=0.5-1 in Na.sub.x CoO.sub.2 may be prepared chemically, using this method. The phase (O3, O'3, P3 or P2) that is obtained is dependent upon the ratio of reactants (stoichiometry) and the reaction temperature. In all four phases, sodium ions are located between sheets of layered (CoO.sub.2)n octahedra. In phases with low sodium ion content, however, the coordination of the ions is trigonal prismatic, and for higher sodium content, the coordination is octahedral, when compounds are prepared via this method.
It has been recognized that because these compounds are mixed conductors, they are good candidates for battery materials. Indeed, electrochemical intercalation and deintercalation of sodium ions occurs with facility at ambient temperatures in batteries with sodium anodes and liquid electrolytes. In the case of the O3, O'3 and P'3 compounds, reversible transitions between the phases occur, resulting in a multi-plateau charge/discharge curve (voltage vs. time for galvanostatic charge or discharge). Thus the fully reduced O3 phase NaCoO.sub.2 with an open circuit voltage of about 2.0 vs. Na is transformed to the O'3 phase as sodium ions are extracted (a Na/NaCoO.sub.2 cell is charged), and then subsequently to the P3 phase as is shown in equation 1. Likewise, O'3 is transformed to the O3 phase upon discharge and the P3 phase upon charge, and the P3 phase is converted to O'3 and O3 upon discharge. EQU O3&lt;=====&gt;O'3&lt;====&gt;P3 (1)
The composition ranges from about x=1 in Na.sub.x CoO.sub.2 (O3 phase) to about x=0.5 (P3 phase) and the voltage vs. Na from about 2.0 to 3.5. This gives a theoretical energy density of 300 Wh/kg for the sodium cobalt bronzes in the O3, O'3 or P3 phases with Na anodes.
The sodium cobalt bronze prepared in the P2 phase (starting composition Na.sub.0.67 CoO.sub.2) also exhibits a multi-plateau charge/discharge curve, but X-ray diffraction studies have shown that no phase transitions occur over x=0.3 to 0.9 in Na.sub.x CoO.sub.2 at room temperature. Rather the plateaus are attributed to electronic effects or to ordering of the Na ions between the layers and the slight shifts or distortions required to accomodate these ions.
Because no major structural changes occur over .DELTA.x=0.6 Na/Co, the P2 sodium cobalt bronze is expected to have better reversibility than the other phases. Cells with Na.sub.3.75 Pb anodes, organic liquid electrolytes and P2 sodium cobalt bronze cathodes have been cycled 300 times or more at current densities of 1-2 mA/cm2, and 1000 cycles to 50% capacity or more have been projected. The slightly greater capacity and higher voltage (up to 4.0 V) give a higher theoretical energy density (440 Wh/kg) for the P2 phase as well, making this sodium cobalt bronze the preferred cathode material for sodium batteries with liquid electrolytes.
While the remarkable reversibility, ease of synthesis, and rather wide capacity range make the sodium cobalt bronzes ideal candidates for sodium battery cathodes, the fairly low gravimetric theoretical energy density is a drawback. In conventional cells, practical energy densities are often less than one-fourth that of the theoretical due, in part, to the rather heavy current collectors needed to contain the liquid. Thus a working Na/P2 Na.sub.x CoO.sub.2 cell with conventional electrolyte would probably have a gravimetric energy density of 110 Wh/kg or less.
In spite of the attractive features of the sodium cobalt bronze system, the use of a sodium anode with an organic liquid electrolyte is widely considered to be a hazard, and has represented a stumbling block to the further development of this type of battery.
Another difficulty is the relatively low practical gravimetric energy density expected for devices based on NaxCoO2 with liquid electrolytes, due to the heavy current collectors (cans or coins) needed to contain the liquid and components.
Furthermore, it is usually necessary to replace sodium with an alloy such as Na.sub.3.75 Pb or a sodium composite electrode in order to improve cycling, due to the tendency of sodium metal to form dendrites in the presence of a liquid electrolyte upon cell charge. This decreases the theoretical energy density further (350 Wh/kg for a Na.sub.3.75 Pb/P2 Na.sub.x CoO.sub.2 cell). Still, the high theoretical volumetric energy densities (estimated to be over 1600 Wh/L for Na/P2 Na.sub.x CoO.sub.2, and 1500 Wh/L for Na.sub.3.75 Pb/P2 Na.sub.x CoO.sub.2) are attractive. FIG. 1 shows theoretical volumetric and gravimetric energy densities of cells with P2 sodium cobalt bronze cathodes and various sodium source anodes for .DELTA.x Na/Co=0.6).